System for Storing Ammonia In and Releasing Ammonia from a Stroage Material and Method for Storing and Releasing Ammonia

ABSTRACT

A system for storing ammonia in and releasing ammonia from a storage material capable of binding and releasing ammonia reversibly by adsorption or absorption for a process with a gradual ammonia demand that can vary over the time. The system has a container capable of housing the ammonia-containing storage material; a heating source arranged to supply heat for the desorption of ammonia from the solid storage medium; and a controller arranged to control the heating source to release ammonia. The heating source is arranged inside the container and surrounded by ammonia storage material. A controllable dosing valve is arranged to dose released ammonia according to the ammonia demand. The controller comprises a feed-forward control arranged to control the heat supplied by the heating source, based on the ammonia demand.

FIELD OF THE INVENTION

This invention relates to ammonia storage, and in particular to a systemand method for storing ammonia in and releasing ammonia from a storagematerial capable of binding and releasing ammonia reversibly byadsorption or absorption.

BACKGROUND OF THE INVENTION

Metal ammine salts which are ammonia absorbing materials can be used assolid storage media for ammonia (see, e.g. WO 2006/012903), which inturn, for example, may be used as the reductant in selective catalyticreduction to reduce NO_(x) emissions, see e.g. WO 1999/01205.

Usually, ammonia is released by thermal desorption, e.g. from metalammine salts, by external heating of a storage container, see e.g. WO1999/01205. The heating elements may also be placed inside the storagecontainer, see e.g. U.S. Pat. No. 5,161,389 and WO 2006/012903.

WO 1999/01205 discloses the use of ammonia as the reductant in selectivecatalytic reduction to reduce NO_(x) emissions of automotive vehicles.The ammonia is released from an either adsorptive or absorptive solidstorage medium, among others Sr(NH₃)₈Cl₂ or Ca(NH₃)₈Cl₂ in granularform, in a storage container and temporarily stored as a gas in a buffervolume. The amount of ammonia to be supplied to a reaction volume in thevehicle's exhaust system is dosed under the control of an electronicengine controller according to the current operating state of the engine(WO 1999/01205, p. 9, last para.). The amount of ammonia to be desorbedfrom the storage medium is controlled by a feedback control in which thepressure in the storage container is measured by a pressure sensor, andif the pressure reaches a pressure threshold, the supply of heat isinterrupted (WO 1999/01205, para. bridging p. 8 and 9).

U.S. Pat. No. 5,441,716 describes a process for rapid absorption cycles(less than 30 minutes) using ammoniated metal halide salts forrefrigerating purposes. A suitable reactor is described having one ormore heat transfer tubes inside that are embedded in the storagematerial. Heat transfer plates are provided to increase the heattransfer from the heat transfer tube(s) into the surrounding storagematerial. The thermal diffusion path lengths and mass diffusion pathlengths are less than 15 mm and 1.5 mm respectively. A similar reactoris described in U.S. Pat. No. 5,328,671.

SUMMARY OF THE INVENTION

A first aspect of the invention is directed to a system for storingammonia in and releasing ammonia from a storage material capable ofbinding and releasing ammonia reversibly by adsorption or absorption fora process with a gradual ammonia demand that can vary over the time. Thesystem comprises: a container capable of housing the ammonia-containingstorage material; a heating source arranged inside the container andsurrounded by ammonia storage material, the heating source beingarranged to supply heat for the desorption of ammonia from the solidstorage medium; a controllable dosing valve arranged to dose releasedammonia according to the ammonia demand; and a controller comprising afeed-forward control arranged to control the heat supplied by theheating source, based on the ammonia demand.

According to another aspect, a method is provided of releasing ammoniastored by storage material housed in a container and capable of bindingand releasing ammonia reversibly by adsorption or absorption for aprocess with a gradual ammonia demand that can vary over the time. Themethod comprises: determining how much heat is to be supplied to theammonia storage material for the desorption of ammonia by means of acontrol comprising a feed-forward control, based on the ammonia demand;supplying the heat by a heating source arranged inside the container andsurrounded by the ammonia storage material; dosing released ammonia bymeans of a controllable dosing valve according to the ammonia demand.

Other features are inherent in the methods and products disclosed orwill become apparent to those skilled in the art from the followingdetailed description of embodiments and its accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of example,and with reference to the accompanying drawings, in which:

FIG. 1 shows an embodiment of an ammonia storage and release system witha storage container in which the ammonia storage material is heatedinternally and the heating source is embedded in the storage material,including a drawing of a suitable shape of an ammonia-containing storagematerial unit to be packed in the container;

FIG. 2 shows different embodiments in cross-section views of storagecontainers in which the heating source is equipped with heat conductingelements wherein FIG. 2 a corresponds to the storage container of FIG.1;

FIG. 3 shows an embodiment in which the heat conducting elements, in theform of fins, are circular plates arranged along an axis of the heatingsource, including a drawing of a suitable shape of an ammonia-containingstorage material unit to be packed in the container;

FIG. 4 shows an embodiment in which the heat conducting fins are made ofporous metal plates to enable desorbed ammonia to flow in the finstowards the exit of the storage container without passing through entireblocks of storage material;

FIG. 5 shows an embodiment similar to FIG. 1, but with a heating sourcesupplied with a hot fluid as a heating medium;

FIG. 6 illustrates the notion of a maximum heat diffusion path length,based on cross-sectional view of the storage container of FIG. 1;

FIG. 7 illustrates by experimental data the reduced delay effects (fastresponse time, enhanced controllability of the pressure of desorbedammonia) of an internal embedded heating source compared with anexternal heating source;

FIGS. 8 and 9 illustrate by experimental data the dosing ability of aninternal embedded heating source compared with an external heatingsource;

FIG. 10 illustrates the feed-forward control of the heat supply,including real-time estimation of the power demand of the heatingsource, based on ammonia demand and observation of temperature;

FIG. 11 illustrates the feed forward-control of the heat supply with anoverlaid feedback control, based on measured pressure in the storageunit;

FIGS. 12 and 13 illustrate other embodiments in which the releasedammonia is not used to reduce NOx, but serves as a fuel for fuel cells.

DESCRIPTION OF EMBODIMENTS

The embodiments pertain to systems and methods for storing ammonia inand releasing ammonia from a storage material capable of binding andreleasing ammonia reversibly by adsorption or absorption for a processwith a gradual ammonia demand that can vary over the time. As describedin applicant's co-pending application WO 2006/012903, metal ammine saltscan be used as solid storage media for ammonia. Thus, the metal-amminesalt constitutes a solid storage medium for ammonia, which represent asafe and practical option for storage and transportation of ammonia.Ammonia is released by thermal desorption from the storage material.

“Gradual ammonia demand” means that the stored ammonia is not demandedall at once, but in a distributed way over an extended period of time(for example, over some hours) with a varying rate, or evenintermittently. The ammonia-containing storage material is held in astorage container from which, in some embodiments, the released ammoniais dosed through a controllable dosing valve in the desired proportion.Between the container and the valve, there is, in some embodiments, abuffer volume.

For mobile units, it is particularly useful to hold the storage material(e.g. metal ammine complex) in a container that can be easily separatedfrom the mobile unit and replaced by a new metal ammine container orrecharged with ammonia in-situ. In one embodiment of replacement ofcontainers, the metal ammine containers are recycled and recharged withammonia in a separate recharging unit or recharging facility.

In some embodiments, the desorbed ammonia is to be used as the reductantin a selective catalytic reduction to reduce NOx emissions, e.g. fromautomotive vehicles, boilers and furnaces. Thus, the system is arrangedto remove NOx from an oxygen-containing exhaust gas of a combustionengine or combustion process. For example, in some of the embodiments, afeed line (which may include the buffer volume) is provided which isarranged to feed released gaseous ammonia from the container directlyinto the exhaust gas in the desired proportion, e.g. dosed by thecontrollable dosing valve. In a reaction volume in the exhaust system, acatalyst is provided for reducing NOx by reaction with the ammonia.

In some embodiments, the combustion engine is a mobile or immobilecombustion engine unit fuelled by diesel, petrol, natural gas, coal,hydrogen or other fossil or synthetic fuel. The NO_(x) to be removed maybe produced by an automobile, truck, train, ship or other motorizedmachine or vehicle, or by a power plant for generating electricity.

The ammonia demand is substantially that amount of ammonia that is ableto remove all the NOx in the exhaust gas; however, if it is nottolerable that any ammonia escapes to the atmosphere, a smallerproportion may be dosed into the exhaust gas to ensure thatsubstantially all the ammonia is decomposed. In some embodiments, theammonia demand is determined based on a measurement of the NOx in theexhaust gas, e.g. measured by a NOx sensor. In other embodiments,information from an engine controller or combustion process controllerabout the operating state is used to estimate the NOx expected in thepresent operating state. For example, the operating state may be definedby the current engine velocity, current load, current drive pedalposition, etc.; knowing these parameters enables the engine controller(or combustion process controller) to determine in real-time theexpected NOx in the exhaust gas. The engine controller is, for example,equipped with a mapping (e.g. in the form of a look-up table) of theentire engine operating area to the corresponding expected NOx emission.Such a real-time predicted NOx signal can be used as an input to thefeed-forward controller to determine the ammonia demand. In someembodiments NOx measurement and NOx prediction, based on the enginecontroller, are combined in order to get a faster, but neverthelessprecise, demand indication; for example, the NOx values predicted by themapping (e.g. look-up table) can be compared with the actual (measured)NOx, and the mapping can be continuously corrected should there be adiscrepancy.

In other embodiments, the desorbed ammonia is to be used, directly orindirectly, as a fuel, e.g. for a power generating unit. For example, insome of these embodiments, the desorbed ammonia is used to producehydrogen in a catalytic ammonia-cracking reactor, and the hydrogen isused as fuel in a fuel cell capable of operating on gaseous hydrogen. Inother embodiments, a fuel for a fuel cell capable of operating onammonia is directly operated with the desorbed ammonia. The gaseousammonia is dosed into the ammonia-cracking reactor or directly into thefuel cell, e.g. by the controllable dosing valve.

In those embodiments, the ammonia demand is substantially that amount ofammonia that has to be provided to the reactor, or the fuel cell, sothat the fuel cell is able to produce the power required.

The heat used in the thermal desorption of ammonia is provided by aheating source. In some embodiments, the heating source is arrangedinside the container such that it is surrounded by, i.e. embedded in theammonia storage material. Unlike a heating source arrangement outsidethe container or inside the container, but at the container wall,substantially all of the heat supplied has to enter the storagematerial. Thus, although a fraction of the heat is nevertheless lost tothe environment, this fraction is smaller than the fraction that wouldbe lost when the heating element were not embedded in the storagematerial.

The heat supply by the heating source is controlled by a controller. Theamount of ammonia to be supplied, e.g. to a reaction volume in avehicle's exhaust system, is, in some embodiments, dosed by a controlledvalve based on the current ammonia demand, e.g. according to the currentoperating state of the engine. Since unloading of ammonia generallyvaries, there will be pressure variations in the storage container (ifthere is a buffer, the pressure variations will also be in the buffer).For example, according to WO 1999/01205 the amount of ammonia to bedesorbed from the storage medium is controlled indirectly based on thepressure variations caused by unloading ammonia from the container, by afeed-back control in which the pressure in the storage container ismeasured by a pressure sensor, and if the pressure reaches a pressurethreshold, the supply of heat is interrupted. By contrast, in some ofthe embodiments of the present invention the controller comprises afeed-forward control arranged to control the heat supplied by theheating source, based on the ammonia demand. This, for example, is thecurrent demand or an estimated future demand, or a combination ofcurrent and future demand. Since the feed-forward control does not onlyreact if the pressure is already too small or too high, the delay withwhich the effective desorption rate is adapted to the rate with whichammonia is unloaded—which is generally a varying rate—is shortened.

The heating controller uses information regarding ammonia deliverydemand and an estimated (model-based) heat loss from the container toensure that the heating source at all times provides an amount of energythat does not allow the unit to cool down below a suitable operationtemperature of the dynamic desorption process. Reaching too lowoperating temperature would result in a desorption pressure below theminimum pressure needed to dose ammonia into e.g. an exhaust line with apressure slightly above atmospheric pressure.

Thus, the feed-forward control is not based on a measurement of how muchammonia has actually been released; rather the amount of heat requiredto release the demanded amount of ammonia is estimated, e.g. by a modelcalculation or by experimental data that links the amount of heatsupplied to the resulting ammonia release. Since the accuracy of such anestimate may be limited, and since the effect of heating (or terminatingthe heating) may only appear after a certain delay, in some embodimentsa feedback control is laid over the feed-forward control, as will beexplained in more detail below.

In some of the embodiments, the feed-forward control cannot only simplyswitch on and off the heating source. Rather, the feed-forward controlis able to adjust the heating source so that it also can supplyintermediate amounts of heat flow between completely on and off; forexample, it is able to adjust the heat flow to the continuousintermediate values in the range between on and off. In someembodiments, the heating source itself can be operated at differentpowers, e.g. by continuously regulating the heating current (in anelectrically powered heating source) or the flow of hot liquid (in anhot fluid heating source). In other embodiments in which the heatingsource can only be operated at maximum power, by fast switching theheating source (e.g. switching the electric supply) with a duty cyclecorresponding to the intermediate value required, an effective amount ofheat corresponding to the intermediate value required is supplied, dueto the thermal inertia of the heating system.

The desorption rate is a function of the temperature and the pressure inthe storage container. To achieve, or maintain, a certain desorptionrate one might therefore think of measuring the temperature, and start,or increase, the supply of heat if the temperature is too low, and stop,or decrease, the supply of heat if the temperature is too low. However,such a temperature based feedback control would have similar delays asthe pressure-based feedback control described in WO 1999/01205.

Generally, the desorption of ammonia from the storage material isendothermic. Thus, desorbing ammonia has a cooling effect. In someembodiments, the feed-forward control is arranged to control the heatsupplied by the heating source such that it compensates the energyrequired for the endothermic desorption of the demanded ammonia from thestorage material. As explained above, this is not (primarily) based on ameasurement of the temperature and a feedback control based on themeasured temperature, but on a calculation (i.e. an estimation) of theendothermic desorption energy required for the desorption of thedemanded amount. Since the desorption energy is proportional to theamount to be desorbed, the required heat energy is, in some embodiments,obtained by multiplying the ammonia demand by the proportionalityfactor.

Even though the heating source is embedded in the storage material, sothat substantially all the heat is absorbed by the storage material, acertain fraction of the heat will be lost to the surroundings throughthe walls of the storage container. In some embodiments, this heat lossis taken into account in the feed-forward control. In these embodiments,the controller is arranged to determine the heat loss of the containerto the surroundings, and the feed-forward control controls the heatsupplied by the heating source such that it compensates the heat loss tothe surroundings. For example, a simple method of estimating the heatloss is based on a model description of the (preferably insulated)storage container in terms of its external surface area (e.g. in m² thatthe heat has to get out of) and a heat transfer coefficient (W/m²K) thatis combined with a temperature gradient from inside the insulation tothe outside. In some of the embodiments, the temperature gradient istaken as the difference between to actual measurements of the internaland external temperatures, or, for example, as the difference of aninternal temperature measurement and an average temperature value of thesurroundings.

In some embodiments, the feed-forward control is such that the heatsupplied by the heating element corresponds to the sum of the desorptionenergy required to desorb the demanded amount of ammonia and the heatloss to the surroundings.

In some of the embodiments in which the heat loss to the surroundings istaken into account in the feed-forward control, the heat loss isestimated on the basis of a temperature measurement. In principle, tocalculate the heat loss, the temperature inside the storage container(or at the inner side of the container wall) and the temperature of thesurroundings (or at the outer side of the container wall) should beknown. Thus, in some embodiments, both the temperature inside thestorage container (or at the inner side of the container wall) and thetemperature of the surroundings (or at the outer side of the containerwall) are measured and used in the heat loss calculation. In otherembodiments, only one temperature measurement is made, and for the othertemperature a (constant) average temperature is assumed, and used in thecalculation (the measured temperature may be the inner temperature, andthe average temperature the outer temperature, or vice versa). In stillother embodiments, no temperature measurement is made, and for both theouter and the inner temperatures average values are used.

As mentioned above, in some embodiments a feedback control is laid overthe feed-forward control of the heat supply. The overlaid feedbackcontrol is based on a pressure measurement in the container. It reducesor terminates the supply of heat by the heating source when the pressureis above an upper pressure threshold, and increases or starts the supplyof heat by the heating source when the pressure is below a lowerpressure threshold. In some embodiments, at overpressure the heat supplyis completely switched off, and at underpressure the maximum heat rateavailable is supplied. There are generally two reasons why an overlaidfeedback control may be useful:

-   -   (i) As explained, the feed-forward control is based on an        estimate of the heat required to desorb the demanded amount of        ammonia; since the accuracy of such an estimate may be limited,        and errors in the estimation may accumulate over time, the        overlaid feedback control provides a sort of error-correction        functionality; and    -   (ii) since the effect of heating (or terminating heating) may        only appear after a certain delay, and the demand can suddenly        significantly increase or decrease, it may happen that the        pressure in the storage container exceptionally undershoots or        overshoots an upper or lower pressure limit.

The overlaid feedback control corrects an accumulated error of thefeed-forward control and constitutes a sort of “emergency intervention”in the case where the pressure becomes too high or low.

The system and process presented here can be used with all storagematerials capable of reversibly releasing ammonia by thermal desorption.These materials may be ammonia adsorbing or absorbing materials.Examples of adsorbing materials are carbon modified with an acid, andzeolites. Examples of absorbing materials are metal ammine salts.

Useful metal ammine salts have the general formula M(NH₃)_(n)X_(z),where M is one or more metal ions capable of binding ammonia, such asLi, Mg, Ca, Sr, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, etc., n is thecoordination number usually 2-12, and X is one or more anions, dependingon the valence of M, where representative examples of X are F, Cl, Br,I, SO₄, MoO₄, PO₄ etc.

During release of ammonia, the original metal ammine saltM(NH₃)_(n)X_(z) is gradually transformed into M(NH₃)_(m)X_(z) with m<n.When all the desired ammonia has been released, the resultingM(NH₃)_(m)X_(z) can usually be converted back into M(NH₃)_(n)X_(z) by anabsorption treatment with an ammonia-containing gas stream due toreversibility of the absorption/desorption process. For several metalammine salts it is possible to release all ammonia and then transformthe resulting material back into the original metal ammine salt in alarge number of cycles.

Typical ammonia contents of the metal ammine complexes are in the rangeof 20-60 wt %, preferably above 30 wt %. As an example, a typical andinexpensive compound, such as Mg(NH₃)₆Cl₂, contains 51.7 wt % ammonia.Using a compaction method such as the one disclosed in applicant'scopending application WO 2006/081824 it is possible to obtain an ammoniadensity within a few percent of liquid ammonia (8-9 bar pressurevessel).

The use of applicant's technology disclosed in WO 2006/081824 enablesstorage of ammonia at significantly higher densities (both on a volumeand a weight basis) than both aqueous ammonia and aqueous ureasolutions. Aqueous urea is an example of a chemical ammonia carrier thatmay provide ammonia for removal of NOx by using a catalyst for NOxreduction and generated ammonia as the reductant.

It is an advantage to deliver ammonia directly in the form of a gas,both for the simplicity of the flow control system and for an efficientmixing of reducing agent, ammonia, with the exhaust gas. The direct useof ammonia also eliminates potential difficulties related to blocking ofthe dosing system which are caused by precipitation or impurities e.g.in a liquid-based urea system. In addition, an aqueous urea solutioncannot be dosed at a low engine load since the temperature of theexhaust line would be too low for complete conversion of urea to ammonia(and CO₂).

The arrangement of the heating embedded in the storage material is in afunctional relationship with the feed-forward control of the heat supplybecause it enables the estimation of the heat to be supplied as afunction of the ammonia demand to be made with better precision.Although this relationship is not mandatory, it is advantageous for thefeed-forward control.

To improve the heat transfer from the embedded heating source to thestorage material, in some embodiments heat conducting elements areprovided that are in thermal connection with the heating source andammonia storage material to increase the internal heat exchanging areabetween the heating source and ammonia-containing storage material.

For example, the heat conducting elements are fins thermally connectedto the heating source and surrounded by ammonia-containing storagematerial.

For example, the heat-conducting elements are made of porous or densealuminium, titanium, stainless steel or similar ammonia resistant metalsor alloys. An example of a suitable metal for the heat conductingelements is aluminium, which is able to tolerate ammonia and salt—unlikee.g. brass. Furthermore, aluminium has a low mass density and excellentthermal conductivity and is thus preferred for efficiently conductingthe thermal energy from the heating element or heating source to thesurrounding storage material kept in the container.

In some of the embodiments, the heating source has an oblong shape. Forexample, the fins are arranged parallel to the heating source'slongitudinal direction. However, in other embodiments, they are arrangedperpendicularly to the heating source's longitudinal direction.

In the latter embodiments, if the desorbed ammonia is to be drawn off atone (or both) of the longitudinal ends of the storage container, thefins could, in principle, be an obstacle to the gas flow (if there is,for example, no other way for the gas to flow around the fins).

In some embodiments, some, or all, of the heat-conducting elements areconstructed of a porous metal structure that passes released ammoniafrom the surface of the storage material being in contact to the fin.This is not only reasonable in the case of fins that could otherwise bean obstacle to the gas flow, but it may also be useful, e.g. withlongitudinal fins, to present “channels” within the storage material forthe desorbed gas to facilitate the gas leaving the storage material.

The heat conducting metal may be made of porous metal plates. Porousmetal sheets/plates/bodies, for example, made of partially sinteredmetal grains will be efficient for heat conduction from the heatingelement (heating source) to the storage material as well as givingincreased gas transport channels from the storage material to thecontainer exit by allowing. ammonia to flow through the porosity of theheating fins. The porosity of the heating fins should be limited so thatthe heat conductivity of the porous metal is at least 10% of theconductivity of the compact metal.

In some of the embodiments, the maximum heat diffusion path length (thedistance from a highly thermally conductive surface to the point of thestorage material farthest away from any highly conductive surface) isabove 15 mm. In some embodiments the mean mass diffusion path length(the arithmetic mean over all ammonia molecules of the shortest distancefrom every ammonia molecule to a gas permeable surface bordering theammonia storage material) is also above 15 mm.

In some embodiments, the heating element is arranged to be powered withelectrical current to produce heat. The heating source may comprise aheat exchanger (or a plurality of heat exchangers) extending into thecontainer. The heat can be obtained from a hot fluid or gas passingthrough the heat exchanger. In some embodiments, the hot fluid or gasis, or is heated by, a hot product gas or fluid from a chemical reactionor a combustion process. In some embodiments the heating source isprovided in the form of one or more heating tubes. The container mayhave a longitudinal extension, and the heating tube or tubes extend(s)in the direction of the container's longitudinal extension.

Thus, this invention relates to the use of storage materials capable ofbinding ammonia by ad- or absorption for the storage and generation ofammonia. For example, solid metal ammine complexes for storage ofammonia and for release of ammonia from the material using controlledinternal delivery of desorption heat directly inside a storage containercan be used. The release of ammonia may be further facilitated byinternal gas channels in the heat exchanging material by using porousmetal structures. Upon release, ammonia may be used as the reducingagent in selective catalytic reduction (SCR) of NO_(x) in exhaust gasesfrom combustion processes.

Other applications using ammonia in mobile or portable units or inspecial chemical synthesis routes where storage of liquid ammonia is toohazardous are also considered embodiments of the present invention. Thisalso includes fuel cell systems where ammonia may be considered anefficient hydrogen carrier as well as other processes consuming ammoniaincluding chemical synthesis routes involving ammonia where storage ofammonia as liquid ammonia is not allowed for safety reasons

The thermal time response of the heated storage medium is, in mostinstances, too slow to fit the ammonia demand in real time (i.e. nearlyinstantaneously). To fit the ammonia demand in real time, thecontrollable dosing valve is provided; it determines the amount ofammonia actually dosed to the outside (e.g. into the exhaust gas).

In some embodiments, the controllable dosing valve is controlled to dosereleased ammonia according to the ammonia demand. For example, thefeed-forward ammonia-demand signal produced by the controller is used tocontrol both the controllable dosing valve and the heating source.Although the response of the thermal ammonia release is relatively fastdue to the feed-forward control of the heating source (compared withfeed-back control schemes), it is still relatively slow compared withthe nearly instantaneous dosing response by the dosing valve. As aconsequence, the amount of ammonia released and the amount dosed maydiffer from each other at certain instances of time. However, if thesame demand signal is used for both the thermal ammonia release andreleased ammonia dosing, these amounts will become equal, due to anaveraging effect, on a time scale comparable to the time constant of thethermal-ammonia-release-by-heating mechanism (assuming that thecalibration of both processes is correct).

As the ammonia amount released instantly not always equals the amountdosed, the pressure in the container housing the ammonia-containingstorage material may vary. In order to ensure that the demanded ammoniaamount is precisely dosed also in view of varying pressures, in someembodiments the demand signal does not directly adjust the controllabledosing valve, but only indirectly, thereby also relying on a mass flowmeter that measure the actual mass flow of ammonia dosed by the dosingvalve. A controller (which may be the controller mentioned above, or adedicated mass flow controller) compares the ammonia demand and themeasured actual mass flow and, based on this comparison, controls thecontrollable dosing valve such that the measured mass flow matches theammonia demand prescribed by the feed-forward ammonia demand signal.

In some of the embodiments with an overlaid feedback control of theheating source to avoid over- and underpressures, the feedback controlsignal is only used to control the heating source, but it is not used incontrol of the dosing valve. Thus, in such embodiments, the controlvalve is always controlled only on the basis of the feed-forward demandsignal, without overlaid feedback signal, while the feed-forward demandsignal also used for the control of the heating source may be overlaidwith the feedback signal. This ensures that the actually dosed ammoniaamount always matches with the demand as close as possible, while under-and overpressures due to the above-mentioned greater time constant ofthe thermal-ammonia-release-by-heating mechanism, and possibly due tocalibration mismatch of thermal desorption and dosing, are avoided.

The embodiments do not only pertain to system (i.e. a product), but alsoto a method of releasing ammonia stored by storage material housed in acontainer and capable of binding and releasing ammonia reversibly byadsorption or absorption for a process with a gradual ammonia demandthat can vary over the time. The method comprises: determining how muchheat is to be supplied to the ammonia storage material for thedesorption of ammonia by means of a control comprising a feed-forwardcontrol, based on the ammonia demand; supplying the heat by a heatingsource arranged inside the container and surrounded by the ammoniastorage material; dosing released ammonia by means of a controllabledosing valve according to the ammonia demand.

In some embodiments of the method, the desorption of ammonia from thestorage material is endothermic, and the feed-forward control controlsthe heat supplied by the heating source such that it compensates theenergy required for the endothermic desorption of the demanded ammoniafrom the storage material.

In some embodiments of the method, the control determines a heat loss ofthe container to the surroundings, and the feed-forward control controlsthe heat supplied by the heating source such that it compensates theheat loss to the surroundings. In some of these embodiments, the heatloss to the surroundings is estimated on the basis of a measurement ofat least one of the temperature inside the container, the temperature ofan inner side of a container wall, the temperature of an outer side of acontainer wall, and the temperature of the surroundings.

In some embodiments of the method, the feed-forward control controls theheat supplied by the heating source such that it compensates both theenergy required for the endothermic desorption of the demanded ammoniafrom the storage material, and the heat loss to the surroundings.

In some embodiments of the method, the control comprises an overlaidfeedback control that, based on a pressure measurement in the container,reduces or terminates the supply of heat by the heating source when thepressure is above an upper pressure threshold and increases or startsthe supply of heat by the heating source when the pressure is below alower pressure threshold.

In some embodiments of the method, NOx is removed from anoxygen-containing exhaust gas of a combustion engine or combustionprocess by feeding released gaseous ammonia from the container into theexhaust gas, and reducing NOx reaction with the ammonia using acatalyst, wherein the control obtains the ammonia demand based on atleast one of (i) a measurement or estimate of the NOx, and (ii)information from an engine controller or combustion process controller.In some of these embodiments, the information from the engine controllerindicates the engine's operational state, and the feed-forward controlestimates the ammonia demand based on the operational state information.

In some embodiments of the method, the desorbed ammonia is used as afuel for a power generating unit.

In some embodiments of the method, (a) the desorbed ammonia is used toproduce hydrogen in a catalytic ammonia cracking reactor, and thehydrogen is used as fuel in a fuel cell capable of operating on gaseoushydrogen; or (b) a fuel cell capable of operating on ammonia is operateddirectly with the desorbed ammonia.

Turning now to FIG. 1, the storage container 1 is heated by a heatingelement 2 representing a heating source placed inside the container 1.In order to dissipate the heat from the heating element 2 there are fins3 representing heat-conducting elements attached to the heating element2. In the example shown, the heating element is powered by electriccurrent. The fins 3 are arranged in planes defined by the longitudinaldirection of the container 1 (i.e. along its cylinder axis) and thecontainer's radial direction. They are suitably made of aluminium orother light materials with high heat conductivity and resistance to theenvironment in the container 1. The ammonia storage material is made inthe shape of blocks 9 to fill out the void in the container 1 (or, inother embodiments, may be put into the unit as powder). The storagematerial 9 is shown both separately and inside the container 1—bothplaces indicated by item 9. When ammonia is released from the solid bythermal desorption, it passes through a tube with an on/off valve 4 to abuffer volume 5. A pressure sensor 10 measures the ammonia desorptionpressure and a dosing valve 6 doses ammonia into an exhaust line 7according to the demand given by a controller 12. The controller 12, forexample, communicates with an engine control unit (ECU; not shown here).An NOx sensor 15 is provided in the exhaust line 7 that delivers an NOxsignal to the controller 12 which, in turns, calculates the ammoniademand to remove the NOx. In other embodiments, the controller 12 gets apredicted ammonia-demand signal from the ECU.

The storage container 1 is insulated by a thermal insulation 8; it alsohas means for measuring the temperature 11 on the outside of thecontainer 1 but underneath the insulation material 8. The controller 12uses the signal from the temperature measurement 11 to estimate/predictthe heat loss through the insulation material. Since most of thetemperature gradient appears at the insulation material 8, thistemperature measurement approximately corresponds to the highertemperature level to be used in the heat loss estimation made by thecontroller 12. The lower temperature level to be used in the heat lossestimation is, in some embodiments, measured by a second sensor, e.g. atthe outer surface of the insulation material 8; in other embodiments aconstant average outside temperature is simply assumed. In someembodiments the heating element itself is constructed with a built-inthermocouple. This may serve both as a security for avoiding overheatingof the heating element and the temperature measurement may also be usedas a parameter in the prediction of the temperature gradient.

This heat loss estimation, combined with the demand to release ammonia,controls the heating input to the heating element 2, in a feed-forwardmanner. Of course, it is actually not necessary to calculate the amountof heat needed to compensate the desorption energy in a two-stepprocedure, in which first the ammonia demand is calculated, and then theheat required to compensate the desorption energy for this demand iscalculated. Thus, in some embodiments, the (measured or predicted) NOxis directly mapped to a number indicating the heat required tocompensate the desorption energy for the amount of ammonia to bereleased to remove the measured or predicted NOx, by a suitable mappingtable or formula. This heat may then directly be combined with theestimated heat loss to obtain the amount of heat to be produced by theheating element 2.

Based on the result of determination, the controller controls theelectric energy delivered to the heat element 2 such that the requiredamount of heat is produced by the heat element 2. For example, it isable to vary the voltage and/or current in a continuous manner,according to the need. In other embodiments, the supply to the heatingelement 2 is permanently switched on and off, with a duty cyclecorresponding to the amount of heat required.

The NOx-containing exhaust gas is produced by a combustion engine orburner, eg. an internal combustion engine 13, and emitted into theexhaust line 7. The NOx sensor 15 is arranged downstream in the exhaustline 7. Further downstream the ammonia, dosed by the dosing valve 6, isdischarged into the exhaust line 7. Still further downstream is anexhaust chamber housing an NOx reduction catalyst 14 capable of removingNOx by reaction with ammonia. The ammonia is dosed such by the dosingvalve 6 that the dosed amount is just sufficient to remove the current(measured or predicted) NOx in the exhaust gas, without any significantamount of ammonia being emitted to the atmosphere.

To this end, in some embodiments the ammonia demand signal (based oncalculation or prediction by the controller 12, as described above) isalso used to control the controllable dosing valve such that it dosesthe released ammonia according to the ammonia demand.

FIG. 2 shows different alternatives (a to c) of applying the concept ofinternal embedded heating of the storage material 9 with large materiallength scales (the latter will be explained in connection with FIG. 6):

-   -   a) The heating element 2 in the form of a rod is placed in the        middle axis of the container 1, here a cylindrically shaped        container. Four storage blocks 9 are placed in the container 1.        The heating fins 3 conduct the heat from the heating element 2        to the storage blocks 9. The container 1 is thermally insulated        by means of the insulation 8.    -   b) The heating element 2 is placed in the middle axis of the        container 1 which is here a rectangular shaped container. Again,        four storage blocks 9 are placed in the container 1, and the        heating fins 3 conduct the heat from the heating element 2 to        the storage blocks 9. The container 1 is insulated, at 8.    -   c) Two heating elements 2 are placed inside a rectangular shaped        container 1. Eight storage blocks 9 are now placed in the        container 1. Again, the heating fins 3 conduct the heat from the        heating element 2 to the storage blocks 9, and the unit is        insulated at 8. Using two internal heating zones may be an        advantage for fast start-up with reduced power demand.

FIG. 3 shows another embodiment in which the internal embedded heatingis arranged in a cylindrical container 1 with a heating element on thecylinder axis 2 and heating fins 3 of a disc-like shape and arrangedperpendicularly to the container's cylinder axis. In this configuration,blocks 9 are, for example, of cylindrical shape with a central hole inorder to fit on the heating rod 2.

In some embodiments, the heating rod 2 has separate internal heatingzones, or “sections”, and each heating disc (or fin) 3 may dissipateenergy to one section (or two neighboured sections) while another zoneis not heated. This can be an advantage, e.g. when lower powerconsumption is desired during start-up, as the system has an ability todirect more energy locally to reach a desired desorption pressurewithout heating the entire storage mass.

FIG. 4 shows a particular configuration in which heating rod 2 isprovided with porous metal sheets acting as heating fins 3, attached tothe heating rod 2 along the longitudinal direction, similar to FIGS. 1and 2. The ammonia released from the storage blocks 9 can then flow inthe container's longitudinal direction through the porous metal sheets 3which may provide faster ammonia release. The heating rod 2 maydissipate the heat through conduction in the porous metal sheet 3. Theporosity of the e.g. sintered metal sheet is below 90% as otherwise theheat conductivity could be too low. In other embodiments,perpendicularly arranged fins, as in FIG. 3, are made of porous metal

FIG. 5 shows an embodiment similar to FIG. 1, but with a heat exchangeras the heat element 2. A hot fluid acting as a heating medium is flowedthrough a central bore in the heating element 2. The heating mediumconveys some of its heat to the surrounding heat element 2, due to heatconduction. The heating medium is, for example, heated by waste heatproduced by an engine (or a burner or chemical reaction chamber etc.)16. A continuously regulable valve 17 arranged in the heating-mediumcircuit is controlled by the controller 12 to adjust (i.e. vary) theflow of the heating medium in such a manner that the heating mediumconveys the amount of heat required to the heat element 2.

FIG. 6 illustrates what is meant by the maximum heat diffusion pathlength, based on cross-sectional view of the storage container ofFIG. 1. In the example shown, the container 1 is a cylinder having acircular cross section with an inner diameter of 10 cm (100 mm). Thestorage material placed in the range of distances in which the distanceto the central heat element 2, i.e. is greater than 15 mm is shown as awhite area at 18 (at the lower left quarter in FIG. 6). Taking also theheat conducting elements 3 into account, the storage material placed inthe range of distances in which the distance to nearest hot surface(central heating element 2 or heat conducting element 3) is greater than15 mm is shown as a white area at 19 (at the lower right quarter in FIG.6). The latter distance is the “heat diffusion path length”. Bycontrast, in the shaded area of the lower right quarter in FIG. 6, theheat diffusion path length is smaller than 15 mm. The maximum of theheat diffusion path length that appears somewhere in the container 2 iscalled the “maximum heat diffusion path length” herein. In the exampleof FIG. 6, the “maximum heat diffusion path length” is greater than 15mm because there is some storage material (namely the storage materialat 19) for which the heat diffusion path length is greater than 15 mm.

Heat diffusion path lengths translate into heat diffusion times. Thus,the smaller is the maximum heat diffusion path length, the shorter isthe delay between the supply of heat and the corresponding release ofammonia. Consequently, in a purely feedback-controlled system, one wouldtend to adopt a design with a small maximum heat diffusion path length,significantly smaller than 15 mm. Since a fast response of the ammoniarelease is not a disadvantage in a feed-forward controlled system,either, in some of the embodiments of the present invention a maximumheat diffusion path length below 15 mm is chosen.

However, it has been recognised that based on a feed-forward control ofthe heat supply one can better cope with such delays. Thus, in someembodiments, the maximum heat diffusion path length is greater than 15mm, e.g. up to 100 mm or beyond. Such a system has a less complicatedinternal structure of fins and is thus be more interesting from anindustrial applicability point-of-view.

FIG. 7 shows delays due to heat conduction in two different experimentsthat are only based on feedback control of the heat supply. In bothexperiments ammonia dosing according to three consecutive driving cycleswith intermediate parking was carried out. Two different types offederally approved driving cycles are used: FTP-75 and US-06 (the latterincludes more high-speed driving). The driving cycles simulate certaindriving conditions. They are characterized by a definition of thevehicle speed as a function of time. When this speed curve isdifferentiated, one can get a dynamic curve showing where a car wouldproduce much NOx (during acceleration) and thus need larger dosing ratesof ammonia.

The experiment starts from a cold unit (room temperature) at t=0. Then agiven amount of power is applied to the heating element to reach anammonia pressure of approximately 2 bars in the buffer. The testingcycle consist of one FTP-75, a “parking period” of 1 hour, a US06driving cycle, 2 hours of “parking” and finally again one FTP-75 cycle.

One setup consists of:

-   -   EXTERNAL HEATING: 2 kg storage material (Mg(NH₃)₆Cl₂) in a        container, external heating element (800 W maximum) wrapped        around the container, insulation material around the heated        container, buffer, pressure sensor, dosing valve (mass flow        controller) and a feed-back control using the pressure as        feed-back measurement, i.e. heating is applied when the pressure        is below the set-point and less (or no) heat is applied when the        pressure is above the set-point. The pressure set-point of the        feedback control above which the heating source is switched off        is 2 bars.    -   INTERNAL HEATING: 2 kg storage material in a container, as        above, but with internal embedded heating element (500 W        maximum), insulation around the container, buffer, pressure        sensor, and dosing valve (mass flow controller). The pressure        set-point of the feedback control above which the heating source        is switched off is 2 bars.    -   Both systems are equally insulated by 3 cm Rockwool.

Looking at the solid curve (EXTERNAL HEATING), this shows the pressure afunction of time during the entire experiment when the experiment iscarried out using an EXTERNAL heating element (as one traditionallywould do). It is easy to see that it takes more than 10 minutes to reacha suitable buffer pressure and when it is finally reached, the thermalinertia of the system causes a dramatic over-shoot of up to 4 bars (alsoduring parking). The next driving cycle starts with a high pressurebecause the over-shoot from the first cycle is not “reduced”. But duringthe next cycle, the feedback control is unable to avoid a largeunder-shoot in pressure (well below the set-point) and a too lowpressure makes it impossible to dose the right amount of ammonia. Thelast cycle is seen to have large oscillations in pressure and also thefirst period of approximate 10 minutes where the pressure is far toolow.

The curve using INTERNAL HEATING is dashed and it can be seen that evenusing lower power (as the internal heating rod has a lower maximum powerlevel) one can reach the desired pressure after cold start much faster.The starting period is only 100-120 seconds as opposed to more than 10minutes using EXTERNAL HEATING. Also, in the remaining period of theexperiment, it can be seen that the pressure is quite stable around the2 bar set-point. Thus, is it demonstrated that rapid start and a morestable system is obtained using the current invention. The total powerdemand during these three cycles is less, as shown below (measured inwatt hours) for the entire 3-cycle experiment:

INTERNAL: 203 W-h

EXTERNAL: 379 W-h

Consequently, using the INTERNAL HEATING, the system performs betterwhile at the same time reducing the power demand by (379−203)1379%=46%.Embodiments described herein (with feed-forward control), for example,use an internal embedded heating of this type.

FIG. 8 shows the accumulated ammonia dosed during the second drivingcycle of FIG. 5 (from 94-104 minutes). This is a US-06 driving cycle andthe assumed ammonia demand amounts to an integral need of approximately7 liters of ammonia gas to be dosed. The figure shows the difference indosing ability using the INTERNAL vs. EXTERNAL heating.

Any increase in the vertical distance between the two curves means thatthe system using EXTERNAL heating cannot follow the demand of ammoniadefined by the assumed driving cycle. Especially in the last 4 minutesof the cycle, almost no increase in the accumulated ammonia dosing curveis seen. The EXTERNAL heating delivers less than five out of the sevenliters needed. The INTERNAL heating follows the driving cycle. Onceagain with regard to FIG. 5, the US06 cycle (taking place approximatelyaround t=100 minutes) for EXTERNAL is seen to have a low pressure in thelast part of the cycle. Thus corresponds to the severe lack of dosing inFIG. 6.

FIG. 9 shows the dosing curve for the final FTP-75 cycle in theexperiment (t=226 to t=257 minutes). Here, the EXTERNAL heating systemonly manages to dose approximately four out of six liters of neededammonia gas. Here it can be seen that it is mainly in the first part ofthe cycle that is difficult to follow. This is also seen in FIG. 5 wherethe pressure curve for EXTERNAL heating is very low in the first 5-10minutes. INTERNAL heating is able to deliver the desired amount ofammonia.

FIG. 10 illustrates the feed-forward control strategy that furtherenables the control of a system for large capacity, including (but notlimited to) systems with long heat diffusion path length scales of thestorage material, e.g. above 15 mm. When the length scales are above 15mm, the time-delay for heat transfer is substantial, even with internalembedded heating. An aim of a control strategy is to avoid reaching astate of “sub-cooling” created by a large release rates of ammonia,which cools the material since the desorption is endothermic. Thiscooling effect is created locally—where ammonia is desorbed—but the newsupply of heat must reach that desorption “front” in the material andthis is potentially far away from the heating source. And when a highammonia release rate takes place when the pressure is above theset-point, in a conventional feedback control, then that will not causethe conventional feedback system to increase the energy input until itis “too late”. Therefore, the feed-forward control algorithm shown inFIG. 10 is advantageous.

Basically, a storage unit needs heat for two things: a) to maintain thetemperature of the system without ammonia being desorbed (compensatingfor heat loss) and b) to supply the necessary amount of heat for ammoniadesorption to avoid cooling of the material.

Thus the elements of the control strategy are:

-   -   a) calculate the heat power needed to compensate for the energy        demand for endothermic ammonia desorption. This is done in real        time using an ammonia dosing demand signal received from the        engine controller (or derived from an expected-NOx signal from        the engine controller) or derived from an NOx measurement by an        NOx sensor; once the demand (for example expressed as a rate n,        in mol/s) is known, the corresponding desorption power        P_(Desorption) can be calculated by:

P _(Desorption) [W]=n[mol NH₃ /s]×ΔH _(NH3,desorp) [J/mol NH₃];

-   -   b) calculate the heat power necessary for compensating for the        heat loss through the insulation material. This is done in real        time using suitable input such as temperature gradient, the heat        transfer coefficient of the insulation layer and the surface        area of insulated system; for example, temperature measurements        provide an internal temperature T_(Cartridge wall) [K] and an        external temperature T_(Outside) [K]; the external surface area        of the storage container is known to be A [m²]; and the heat        transfer coefficient of the container's insulation is known to        be h [W/K/m²]; then the power required to compensate the heat        loss can be calculated by

P _(Heat loss comp) =A×h×(T _(Cartridge wall) −T _(Outside))[W];

-   -   c) add a) and b) in real time to predict the total power demand        P_(total) at:

P _(total) =P _(Heat loss comp) +P _(Desorption;)

-   -   d) control the heat element so that it supplies the total power        demand P_(total).

In more practical terms: if one accelerates the car dramatically, thecontrol system immediately adds more heat to the storage unit even ifthe pressure is actually slightly above where the set-point would be ina conventional feedback control. This avoids a short period of ammoniadeficiency that would show up in a conventional feedback system.

If the surface area, A, and the heat transfer coefficient, h, of thecontainer are not known a priori, the controller may also comprise analgorithm that estimates the heat-loss parameters during e.g. a periodof 10 minutes of system operation. The coupling of the knowledge ofamount of heat input and the amount of released ammonia over a specificperiod of time (e.g. 10 minutes) will enable the controller to estimatethe value of A×h (if the temperature gradient is known). It will not beable to estimate the value of two parameters, A and h, independently butthe description of the heat loss as a function of temperature gradientwill generally be sufficient if the value of A×h is known.

FIG. 11 shows an overlaid feedback control to provide an additionalsafety feature of the control system. The pressure scale shown indicatesdifferent pressure levels in the pressure control strategy.

The basis pressure is the atmospheric pressure of the surroundings. Thepressure in the exhaust line is slightly higher, e.g.P_(Exhaust line)=1.2 bar. Dosing of ammonia is not possible unless thedosing valve gets a certain supply pressure from the buffer ofP_(Minimum) (as an example say 1.5 bars). The normal set-point isP_(NH3, setpoint) (e.g. 1.8 bars). The control strategy presented inFIG. 8 might only be active in the pressure range betweenP_(NH3, setpoint) and P_(Heat-off) (e.g. 2.2 bars). Above a certainpressure (P_(Heat-off)), the heat is turned off at any rate as a safetyfeature. P_(Heat-off) is higher than the set-point in a conventionalfeedback control would be, since it is a safety feature, but the“normal” control is performed by the feed-forward part. AtP_(Satefy max) an optional pressure relief valve will open to avoid anypressure above a mechanical design level.

When the pressure is below P_(NH3, set-point), then maximum heatingshould be applied (unless the car is not able to deliver that much powerin the current state of engine load). P_(NH3, set-point) is lower thanthe set-point in a conventional feedback control would be, since alsothis is a safety feature, but the “normal” control is performed by thefeed-forward part.

FIGS. 12 and 13 illustrate other embodiments in which the releasedammonia is not used to reduce NOx, but serves as a fuel for fuel cells.In the embodiment of FIG. 12, ammonia stored in a container (1) instorage material (9) is released by a heater (2) based on a feed-forwardcontrol of the heat supplied, as explained in illustrated in threeprevious figures. The released ammonia is supplied to a catalyticcracker (20); the produced hydrogen is fed to a fuel cell (21 a) capableof converting hydrogen to electricity. In the embodiment of FIG. 13 thereleased ammonia is directly supplied to a fuel cell (21 b) capable ofdirectly converting ammonia to electricity.

The feed-forward control strategy of FIG. 10, with an optionalcombination with the pressure level strategy of FIG. 11, constitutes acontrol strategy that can handle the long time-delays of operatingcombined with a safe ammonia storage system using endothermic ammoniadesorption from storage units with large material length scales above 15mm. The strategy of FIGS. 10 and 11 is well-suited for the concept ofinternal heating as the heat-compensating term is easier to compute. Onereason is that while the temperature of the internal heating elementwill typically fluctuate quite substantially, the temperature of thecontainer wall will almost be constant over longer periods of time—andtherefore the temperature gradient to the surroundings does not changerapidly. If an external heating was applied, the temperature gradient tothe surroundings would change very dynamically because the temperatureof the container wall would increase and decrease with every initiationand ending of a heating period.

1. A system for storing ammonia in and releasing ammonia from a storagematerial capable of binding and releasing ammonia reversibly byadsorption or absorption for a process with a gradual ammonia demandthat can vary over the time, comprising: a container capable of housingthe ammonia-containing storage material; a heating source arranged tosupply heat for the desorption of ammonia from the solid storage medium;a controller arranged to control the heating source to release ammonia;a controllable dosing valve arranged to dose released ammonia accordingto the ammonia demand; wherein the controller comprises a feed-forwardcontrol arranged to control the heat supplied by the heating source,based on the ammonia demand.
 2. The system of claim 1, wherein thedesorption of ammonia from the storage material is endothermic, and thefeed-forward control is arranged to control the heat supplied by theheating source such that it compensates the energy required for theendothermic desorption of the demanded ammonia from the storagematerial.
 3. The system of claim 1, wherein the controller is arrangedto determine a heat loss of the container to the surroundings, and thefeed-forward control is arranged to control the heat supplied by theheating source such that it compensates the heat loss to thesurroundings.
 4. The system of claim 3, wherein the heat loss to thesurroundings is estimated on the basis of a measurement of at least oneof the temperature inside the container, the temperature of an innerside of a container wall, the temperature of an outer side of acontainer wall, and the temperature of the surroundings.
 5. (canceled)6. The system of claim 1, wherein the controller comprises an over-laidfeedback control arranged, based on a pressure measurement in thecontainer, to reduce or terminate the supply of heat by the heatingsource when the pressure is above an upper pressure threshold and toincrease or start the supply of heat by the heating source when thepressure is below a lower pressure threshold.
 7. The system of claim 1,arranged to remove NOx from an oxygen-containing exhaust gas of acombustion engine or combustion process, further comprising: a feed linearranged to feed released gaseous ammonia from the container into theexhaust gas, a catalyst for reducing NOx by reaction with the ammonia,and wherein the controller is further arranged to obtain the ammoniademand based on at least one of (i) a measurement of the NOx, and (ii)information from an engine controller or combustion process controller.8. The system of claim 7, wherein the controllable dosing valve isarranged to dose released ammonia into the exhaust gas.
 9. The system ofclaim 7, further comprising an NOx sensor, on which the NOx measurementis based.
 10. The system of claim 7, wherein the information from theengine controller is indicative of the engine's operational state, andthe feed-forward control is arranged to estimate the ammonia demandbased on the operational state information.
 11. (canceled) 12.(canceled)
 13. (canceled)
 14. The system of claim 1, arranged to use thedesorbed ammonia, or a derivative of it, as fuel in a fuel cell, furthercomprising: (a) a catalytic ammonia cracking reactor to producehydrogen, a fuel cell capable of operating on gaseous hydrogen; or (b) afuel cell capable on running directly on released ammonia.
 15. Thesystem of claim 1, wherein the ammonia storage material is capable ofbinding and releasing ammonia reversibly by absorption and is a chemicalcomplex in the form of an ionic salt of the general formula:M_(a)(NH₃)_(n)X_(z)1 wherein M is one or more cations selected fromalkali metals, alkaline earth metals, and transition metals orcombinations thereof, X is one or more anions selected from fluoride,chloride, bromide, iodide, nitrate, thiocyanate, sulphate, molybdate andphosphate ions, a is the number of cations per salt molecule, z is thenumber of anions per salt molecule, and n is the coordination number of2 to
 12. 16. (canceled)
 17. The system of claim 15, wherein the ionicsalt is either chloride or sulphate salts of Mg, Ca, Sr or mixturesthereof.
 18. The system of claim 1, wherein the ammonia storage materialis in the form of shaped units of ammonia storage material.
 19. Thesystem of claim 18, wherein the ammonia storage material is compacted toa dense block, rod, cylinder ring or edged unit with a density above 70%of the theoretical maximum skeleton density of the saturated solidmaterial.
 20. (canceled)
 21. (canceled)
 22. (canceled)
 23. (canceled)24. (canceled)
 25. (canceled)
 26. (canceled)
 27. (canceled) 28.(canceled)
 29. (canceled)
 30. (canceled)
 31. The system of claim 1,wherein the controllable dosing valve is also feed-forward controlled todose released ammonia according to the ammonia demand.
 32. The system ofclaim 31, wherein an ammonia-demand signal by the controller is used tofeed-forward control both the controllable dosing valve and the heatingsource.
 33. The system of claim 1, wherein control of the controllabledosing valve comprises a mass flow meter that is arranged to measure themass flow of ammonia dosed by the dosing valve.
 34. The system of claim33, arranged to compare the ammonia demand and the measured mass flow,and control the controllable dosing valve such that the measured massflow matches the ammonia demand.
 35. A method of releasing ammoniastored by storage material housed in a container and capable of bindingand releasing ammonia reversibly by adsorption or absorption for aprocess with a gradual ammonia demand that can vary over the time,comprising: determining how much heat is to be supplied to the ammoniastorage material for the desorption of ammonia by means of a controlcomprising a feed-forward control, based on the ammonia demand;supplying the heat by a heating source; dosing released ammonia by meansof controllable a dosing valve according to the ammonia demand.
 36. Thesystem of claim 1, wherein the heating source is arranged inside thecontainer and is surrounded by ammonia storage material.
 37. The methodof claim 35, wherein the heating source is arranged inside the containerand is surrounded by ammonia storage material.